Pneumatic positioners generally include: a thin walled casing composed of a shell-like bottom part and a shell-like cover part; a membrane or diaphragm plate which is connected by means of a gas-tight membrane or diaphragm to the side walls of the casing halfs so as to be movable back and forth in the casing; a drive shaft or spindle which is connected to the membrane plate and which is disposed in a sealing relationship through the bottom part of the casing; gas feed connecting pieces disposed within the bottom part and/or in the cover part; and a set of casing coil springs abutting, on one hand, the membrane plate and, on the other hand, the bottom part or cover part of the casing so as to produce a biasing force.
Such pneumatic positioners are well known to those skilled in the art. They are used, for example, to actuate valves, slides and other linkages. It is a common practice in the use of such pneumatic positioners to adjust the effective restoring force or biasing force either by changing the number of coil springs arranged in the casing between membrane plate and/or cover part or bottom part, or by changing the spring constant of the coil springs. This practice is inefficient in that an inventory of coil springs has to be kept in readiness in a location separate from those used within the positioners. This is especially burdensome if many positioners are in use throughout one's plant; considerable storage space would have to be provided. Moreover, there is always present the danger that the coil springs stored outside the pneumatic positioner will be lost or misplaced. Thus, it is more than desirable to have a pneumatic positioner whose spring bias may be easily adjusted without having to physically replace the coil springs arranged in the positioner's casing.